Three-Way Catalyst (TWC) compositions are polyfunctional, in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides in a gaseous stream. Such catalyst compositions find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile, truck and other gasoline-fueled engines.
“Close-coupled” catalysts are generally defined as located in the engine compartment, typically less than one foot, more typically less than six inches from, and commonly attached directly to, the outlet of the exhaust manifold. “Medium-coupled” catalysts are also known in the prior art and are generally defined as located (downstream of any close-coupled catalyst) usually not more than about twenty-four, typically eighteen, inches from the outlet of the exhaust manifold. Underfloor catalyst members are also known in the prior art and are located (downstream of any close-coupled and/or medium-coupled catalysts) under the floor of the vehicle adjacent to or in combination with the vehicle's muffler.
Motor vehicle exhaust treatment devices such as catalytic converters are conventionally located in underfloor position in the vehicle. For the purposes of the present invention, the term “vehicle” is to be understood as signifying a passenger car or truck and the term “engine” is to be understood as signifying a gasoline-powered internal combustion engine associated with the vehicle.
By the time engine exhaust gases travel from the outlet of the exhaust manifold through an exhaust pipe to a catalytic converter, the gases cool significantly relative to the temperature at or near the manifold, so that there is a significant period of a low rate of conversion of the pollutants in the exhaust gas stream before the exhaust gases heat the catalyst in the catalytic converter to its light-off temperature. Accordingly, during the cold start period of engine operation, there is a significant discharge of engine exhaust gas containing a relatively high amount of pollutants.
To reduce the level of pollutants in the exhaust gas stream, particularly the level of hydrocarbons and carbon monoxide, an air pump used in conjunction with the engine, can aid in the oxidation of such pollutants. However, vehicle manufacturers prefer to avoid using mechanical pollution control devices such as air pumps which, with their associated plumbing and mechanical parts, affect the engine architecture and are difficult to control without having an adverse impact on the optimum performance of the engine. Accordingly, vehicle manufacturers prefer to tune the engine for optimum performance without using mechanical types of a pollution control device and instead meet the vehicle emission standards discussed below solely with the use of catalyst members comprising one or more upstream catalyst bricks of the close-coupled and/or medium-coupled type and, if necessary, a catalytic converted located in an underfloor position. Increasingly stringent governmental emission standards require, however, that cold-start emissions be reduced.
The current “LEV” (low emission vehicle) standards in effect for all states other than California prohibit vehicle emissions above 0.08 gram of non-methane hydrocarbons per mile, 3.4 grams of carbon monoxide per mile and 0.2 gram of NOx (nitrogen oxides) per mile. Many vehicle manufacturers have difficulty in meeting the current standards solely with the use of available upstream and/or downstream catalyst compositions without the concurrent use of additional mechanical devices such as air pumps. Of even greater concern is the fact that the California Air Resource Board (“CARB”) has promulgated new “ULEV” (ultra-low emission vehicle) standards that will prohibit vehicle emissions above 0.04 gram of non-methane hydrocarbons per mile, 1.7 grams of carbon monoxide per mile and 0.2 gram of NOx per mile. Moreover, based on historical trends in vehicle emission standards, it is likely that the new ULEV standards will be required nationwide within a few years. Unless an effective method of meeting the new ULEV standards can be rapidly developed and implemented, vehicle manufacturers face the difficult problem of achieving such standards without significant changes in engine/exhaust architecture, incorporation of additional mechanical pollution control devices and the use of large amounts of expensive precious metal-based catalyst systems.
A typical motor vehicle catalyst is an underfloor TWC which catalyzes the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum or palladium, rhodium rhodium and iridium) located upon a high surface area, refractory oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
Therefore, a need exists in the art for catalytic materials which are effective at lower operating temperatures and which utilize smaller amounts of platinum group metal components.